ENRICHMENT PROCESSES IN THE SUB-ARC MANTLE - A SR-ND-PB ISOTOPIC AND REE STUDY OF PRIMITIVE ARC BASALTS FROM THE PHILIPPINES

Within the Macolod Corridor, a SW-NE-trending extensional zone cross-c utting the Taiwan-Luzon are in the Philippines, numerous centers of er uption have produced primitive to moderately evolved basalts. Those fr om the eastern part of the area (Macolod basalts) are characterized by increasing LREE/HREE with decreasing HREE. Whereas such a relationshi p might reflect variable degrees of melting of garnet peridotite, decr easing Nd-143/Nd-144 values with increasing LREE/HREE suggest that thi s feature is inherited from the source. Clear correlations exist among Nd-143/Nd-144, Sr-87/Sr-86, and LREE/HREE values, but there is no cor relation between these data and Pb isotopic compositions. In addition, LREE enrichment is accompanied by HFSE enrichment. These relationship s are interpreted to reflect infiltration of the source by melts (cont rolling the REE and HFSE geochemistry). Larger contributions from the slab, indicated by high LREE/HREE Values and HFSE concentrations, are considered to result in higher degrees of partial melting and, hence, lower HREE concentrations.